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ERS Meter

Anyone here have experience with this ESR Meter? I'm thinking of buying one to test for bad caps. I'd like to be able to test in circuit and this one claims that it can. Are there limitations when testing a capacitor in circuit? Any recommendations greatly appreciated. I don't want to spend more than 50 bucks.

An ESR meter won't tell you the in-circuit capacitance of a device, just its series resistance. It can be useful if you know how to use it and know what the readings mean.

There are some caveats--you don't use one on a powered circuit and if there are several capacitors hooked in parallel (common on computer PSUs), it can only measure the composite ESR, not that of any particular cap.

Basically, a low-voltage (so as not to involve semiconductor bandgap) AC signal is applied and the current flow is measured. All the other stuff is refinement and window-dressing. If you're of a mind to "roll your own", here's a popular DIY ESR meter project.

The whole theory hinges on the fact that wet electrolytics dry out with age, which causes the effective internal resistance to rise, which also (remember I2R?) causes self-heating to increase and accelerate aging.

An ESR tester is useful for a 'go/no go' type of test, but if you're looking at accurately measuring the ESR of a 68,000 uF electrolytic in-circuit, forget it. Remember that in-circuit ESR testers put out only a couple hundred millivolts AC, which would make the AC drop across a big low-ESR capacitor in the microvolt range.